Low-pressure plasmas (LPPs) have been widely investigated and have found a number of applications in semiconductor manufacturing and processing. A relatively large and uniform plasma has to be sustained and confined in a vacuum system in order to achieve a uniform processing rate across the whole chamber. The benefit of LPP is that it requires a low breakdown voltage to ignite and has a relatively high electron temperature and low neutral temperature. However, a vacuum system is required to generate a plasma in a low pressure environment. Vacuum systems, including the chamber, pumps and other related components inside the chamber, can be expensive and difficult to adapt to changes in application requirements. Semiconductor processes, such as plasma etching and plasma deposition, can also create contamination in the vacuum chamber that may require constant cleaning, repair and maintenance.
Evaporation has also been widely used to make coatings on materials. A typical evaporator uses a small electron beam to heat the surface of the material which is to be used as the coating until it evaporates. Simply heating a crucible is another way of raising the temperature high enough such that sufficient material enters the vapor phase. These techniques also require a vacuum system since the electron beam, and more importantly the evaporated vapor, would otherwise have so many collisions in the background gas that they could not reach their intended targets.
Atmospheric pressure plasmas can be categorized into several types, according to the configuration. First is the corona discharge, which is usually ignited by applying a DC voltage (˜10 kV) between a point electrode and a plane electrode. The distance is at the scale of several mm, and the current is usually kept low (<300 μA) to prevent arcing. The second is the dielectric barrier discharge (DBD), which is usually generated between two metal electrodes, where one or both are coated with a dielectric layer and may have a spacing of several millimeters. Generation of DBDs in general requires a 10 to 20 kV DC voltage, and the plasma can be spread relatively evenly in a large area. Finally, there is the atmospheric pressure plasma jet (APPJ). It may include two concentric electrodes, where the inner one may be applied with a 13.56 MHz RF power or a 2.45 GHz microwave power. Gases with adjustable rates are introduced between the two electrodes during the discharge. The ignition condition for an APPJ can be easily achieved, and the discharge of an APPJ may be homogenous, volumetric and low in gas temperature. However, sustaining a gas discharge at atmospheric pressure may be more difficult than in a vacuum chamber, since time constants for instabilities decrease with increasing pressure. A simple approach to generate large-volume atmospheric-pressure plasmas may be to create a large electric field around the cathode boundary region to supply sufficient production of electrons, which may depend on the specific structure of the electrodes and type of feed gas.
A comparison of the breakdown voltage and electron density of different atmospheric-pressure plasmas and low-pressure plasmas are listed in Table 1. It can be seen that APPJs have similar breakdown voltage to the low pressure discharges, which can be one to three orders of magnitude lower than the other atmospheric-pressure discharges. At the same time, the electron densities of APPJs are also in the same range of low-pressure discharges, but lower than the rest of atmospheric pressure discharges.
TABLE 1Breakdown voltage and electron density of plasma discharges.Electron densityPlasma SourceBreakdown voltage (kV)(cm−3)Low pressure discharge0.2-0.8 108-1013Arc10-501016-1019Corona10-50 109-1013DBD 5-251012-1015APPJ0.05-0.2 1011-1012